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Tailored Dual-phase Steels for Axial and Lateral Load Cases With their balanced property profile dual-phase steels differentiate positively from the features of complex-phase and retained-austenite steels. Thyssenkrupp shows the advantages of cold-rolled steel grades for lightweight design and crash-relevant structural parts for body development. New grades increasingly offer opportunities to make parts in keeping with the materials philosophy of the OEM in question using both cold and hot forming.
Several Development Strands With around 400 billion euros in sales and almost 800,000 employees, the auto sector is Germany’s biggest industry – and it is undergoing a massive transformation. Beyond mere vehicle production, car manufacturers are evolving into mo bility service providers, while at the same time internet companies, for example, are moving into automobile production. Boundaries are shifting. From autonomous driving to hybrid cars to electric mobility, automobile manufacturers have to pursue several development strands simultaneously. At the same time, this requires huge investment of high cost pressure. It is a trend that presents material producers too with challenges. It seems clear that the focus of
Author DR.ING. PATRICK KUHN is Product Manager Advanced High-strength Steels at Thyssenkrupp Steel Europe AG in Duisburg (Germany).
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development will shift sharply towards part cost optimization, but without compromising occupant and pedestrian protection.
Family of Advanced High-strength Steels Thyssenk r upp has been suppor ting customers in the auto industry for three decades, building up competence and experience in a large number of projects. To continue supporting OEMs with tailored, cost-efficient materials the company has broaden its family of cold-rolled dual-phase steels far beyond the previous high-strength lightweight design steel grades. Among the advanced high-strength steels, dual-phase steels provide particular advantages thanks to their attractive combination of high strength, good cold forming properties and good joining characteristics – and this despite generally moderate alloying. This balanced overall property profile differentiates them for example f rom t he complex pha se steels a nd retained-austenite steels and gives them a dominant role in the field of crash-relevant structural parts – a factor that has encouraged Thyssenkrupp to expand its dualphase steel product line.
Dual-phase Steels as a Made-to-measure Solution An analysis of the specific loads acting on parts in the body-in-white was the starting point for the new dual-phase steel line-up. What loads are most relevant in the event of a crash? Is bending stress more important for example or do tensile loads or compressive stresses matter most? Following on from these considerations it was important to see things from the OEM perspective and establish their key selection criteria for increasing performance and leveraging weight-reduction potential in each part. With regard to steel development, especially precision work in metallurgy, it was then important to transfer these criteria into material properties and binding specifications. As a result, alongside the classic dual-phase steels in the strength range (yield strength) between 500 and 1200 MPa – the so-called universal or standard grades – additional variants are now available, allowing particular application-related requirements to be met more precisely. For customers this means they can further optimize part manufacture and geometry producibility or further improve key part functions such as crash safety. Improved crash properties can be achieved for example by raising yield strength by approximately 100 to 120 MPa
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versus the standard grade. Side impact energy is absorbed over a shorter intrusion distance, allowing a further cost and weight reduction and increased passenger safety. Figure 1 shows an overview of multiple application options for these DP-K dualphase steels [1].
Axial Load Cases: Application Example Front Side Beams The potential offered by the extended product line can be illustrated by a concrete part example. Taking a front side beam for example and using a conventional high-strength micro-alloyed HX 340 grade as a reference, Table 1, the dual-phase steels in the 600-MPa strength class already offer substantial advantages: The use of the DP-K330Y590T universal grade can save 13 % weight. For each side beam this means a reduction in part weight from 4.7 to 4.1 kg and at least a cost saving of 1.2 %. If greater forming reserves are required, either to optimize part geometry or to reduce failure costs in the press shop through greater manufacturing reliability, a solution using the elongation-optimized DP-K330Y590T-DH grade commends itself. The weight saving of 13 % remains. With an elongation-optimized variant in the 800MPa strength class, the DP-K440Y780T-DH, part geometries and crash characteristics previously unattainable in this strength class come within reach. Applied to the reference side beam with a slight simplification of the geometry, this means a further significant mass reduction down to 3.69 kg part weight; a saving of 22 % versus the origi nal microalloyed design.
Lateral Load Cases: Application Example Sill The wider product portfolio also provides new solutions and optimized material combinations for further crash-relevant parts laterally at the vehicle. A particular highlight are the dual-phase steels with elevated yield strengths, for example in the 1000- and 1200-MPa strength class. They already border on the performance of classic hot-forming grades and display their strengths through
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Outer skin applications e.g. as sheet in Z und ZM Ecoprotect and in DP-K290Y490T
Axial load cases e.g. as side beam in DP-K330Y590T and DP-K440Y780T Additional requirements to part complexity or axial crash are fulfilled by DH variants
Lateral load cases e.g. parts in DP-K420Y590T, DP-K700Y980T and DP-K900Y1180T Increased deformation resistance and intrusion protection in lateral crash
FIGURE 1 Overview of multiple application options for dual-phase steels – for shell, structure and crash (© Thyssenkrupp)
their high deformation resistance in applications such as side impact beams, sills, and roof cross beams, where high intrusion protection and a balance between reduced weight and improved passenger safety are particularly important. A sill in the standard variant of the 1200-MPa class, the DP-K780Y1180T steel, already saves 8 % weight versus the reference, a 1000-MPa dual-phase steel: Instead of
ence materials, also in terms of hole expansion and bending behavior.
Cold and Hot Forming Growing Together The advances in the 1200-MPa strength class were driven in particular by collaborations with Asian OEMs. Japanese carmakers, for example, are seeking to
Side impact energy is absorbed over a shorter intrusion distance. 7.2 kg the part now weighs 6.6 kg. A cost reduction can be achieved as well. A further 4-% weight saving to 6.3 kg part weight is possible using the variant with elevated yield strength.
Good Forming Properties The 1200-MPa dual-phase steels display good forming properties thanks to favorable strain hardening behavior, for example in deep drawing. With the limiting dome height test, Figure 2, a hemispherical tool is pressed into a round metal sheet which is fixed at its edges. The values achieved in forming tests are close to those of their refer-
implement their philosophy, strongly focused on the use of ultrahigh-strength cold-forming steels, in Europe as well, with the help of strong local partners. As a result of these developments there is an increasing number of parts that can be made by both cold and hot forming. The Japanese example shows the advantage of targeting the further development of cold and hot forming strategies so as to significantly increase the number of parts that can be processed both hot and cold. The more materials available that offer the key properties for crash-relevant parts – energy absorption, deformation resistance and part complexity – in both cold and hot
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TABLE 1 Three DP-K steel options for the front side beam compared to the conventional reference steel HX340LAD (© Thyssenkrupp) –
Material
Sheet thickness [mm]
Part weight / percentage weight reduction
● Reference
HX340LAD
2.30
4.72 kg
● Option 1
DP-K330Y590T
2.00
-13 %
● Option 2
DP-K330Y590T-DH
2.00
-13 %
● Option 3
DP-K440Y780T-DH
1.80
-22 %
forming, the greater the choice for customers and users to find the optimum part solution. The ability to anticipate the changes caused by the transformation in the automotive industry and, together with the OEMs, to develop concepts for the changed conditions and new fields of application will be a key factor. This can also mean completely new concepts such as the composite tribond, which Thyssenkrupp now offers as a production material for hot forming and for parts with particular requirements. One
current application where a differentiated port folio of high-performance dual-phase steels can offer made-to-measure, costefficient solutions is battery housings for electric cars. This calls for property profiles that meet very high intrusion protection requirements and are still suitable for the mass market under high cost pressure. Thyssenkrupp has carried out comprehensive part analyses to identify the material concepts required for future electric powered vehicles. The family of dual-phase steels will play a central role in this. |
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[1] Thyssenkrupp: Dualphasen-Stähle in neuer Vielfalt. PDF-Download. Online: https://www.Thyssenkrupp-steel.com/de/produkte/feinblech-oberflaechenveredelte-produkte/mehrphasenstahl/ dualphasenstahl/#, access: September 6, 2017
FIGURE 2 Limiting dome height test for deep drawing
40 Maximum drawing depth reached [mm]
Reference
of steel sheets – maximum draw depth reached in mm (© Thyssenkrupp)
35 30 25 20 15 10 5 0 DP-K330Y590T-GI
DP-K440Y780T-GI
DP-K330Y590T-DH-GI
DP-K440Y780T-DH-GI
On 1.5 mm thick test sheets, mean value: 2-times standard deviation
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